The present invention concerns new compositions for soft soldering, of lead-based alloys with tin, bismuth, cadmium and antimony. In soft soldering with tin-lead solders the interposed metal has normally a melting point between about 180.degree. to 260.degree. C.
The solder should adhere strongly to the metallic surfaces of the parts to be joined, by "wetting" them. This is readily seen as the spreading of a drop of molten solder upon contact with such a surface. Simultaneous with the wetting, the tin in the solder reacts with the wetted base metal, forming a layer of intermetallic compounds. Such intermetallic compounds are brittle and care must be taken (i.e. by avoiding excessive or prolonged heating) to prevent this layer from becoming too thick and thus weaken the joint. Table 1 given hereafter shows the dependence of the thickness of the said layer upon holding time at 310.degree. C., as measured in a straight 20 wt % tin-lead solder alloy over copper. The thickness growth depends upon tin supply from the solder, thus our main aim of lowering the tin content will result in reducing the intermetallic growth. This will also reduce the solder price (tin is the higher priced component), a great benefit by itself.
Table 1: Intermetallic layer thickness after holding for various times at soldering temperature (310.degree. C.):
______________________________________ Time Thickness (sec) (.mu.m) ______________________________________ 2 0.75 4 0.80 8 1.00 16 1.2 32 1.3 64 1.6 ______________________________________
The wetting of commonly soldered metals is promoted by tin; but tin is relatively high priced and has low mechanical strength. By adding lead its mechanical properties improve and at the eutectic composition the alloy has a convenient melting point of 183.degree. C.
Tin lead eutectic solder (63 wt % Sn) is mandatory for work where exceptionally high quality is required, as for example in high-tech electronics. This alloy can clearly not be substituted for his unique solidification properties. An almost eutectic alloy (60 wt % Sn) is commonly used whenever the difference between it and the eutectic does not impaire the quality of the joint to be made. An alternative soldering alloy has been proposed to replace this near-eutectic composition by U.S. Pat. No. 3,945,556 (Manko): 50-57.5% tin, 1.5-4% antimony and the balance lead. It is seen that the savings are not big (replacing 2.5-10 wt % tin by 1.5-4 wt % antimony), nevertheless the alloy is to be accepted as a possible alternative. Not being a eutectic composition there is a gap between the "solidus" temperature (at which the alloy begins to melt) and the "liquidus" temperature (at which the alloy is completely liquid). This solidus to liquidus temperature range (mushy zone) .DELTA.T is, thus, an indicator of an alloy's quality as a solder. Different applications require different mushy zone widths. For most practical purposes in mechanical or electrical assemblies, usage is made of cheaper alloys with a tin content of 30-50 wt % tin having the properties listed in Tables 2, 3 and 4. Our efforts were devoted to develop suitable substitutes to these alloys and offer them to usage with the relevant properties characterized.
There are a number of patents concerning alternative lead-based soldering alloys containing cadmium, bismuth and antimony besides and instead of part of the tin in tin-lead solders:
U.S. Pat. No. 1,301,688 (Gurevich) claims alloys containing 5-20 wt % cadmium, up to 20 wt % tin, balance lead. His preferred composition (10 wt % cadmium, 10 wt % tin, balance lead) is indeed cheap as claimed, but is also a very low-grade solder with a liquidus-solidus gap of 105.degree. C.
U.K. Pat. No. 464,824 (Gohringer) claims for solder alloys comprising up to 3 wt % cadmium, 0.5-5 wt % bismuth, 0.5-5 wt % antimony, 0.5-23.5 wt % tin with balance lead. These compositions do not melt in the range of temperatures of our target tin-lead solders (from 30 wt % tin and up) and can be useful only to special soldering applications where a particularly high liquidus-solidus gap is preferred (higher than 95.degree. C.) by minimizing their flow properties.
Gohringer (Swiss Pat. No. 221,997) claims alloys having from 0.1 to 0.75% bismuth, 1.5-3 cadmium and no antimony. His formulations are liable to have (according to our experience), a melting temperature gap of about 90.degree. C., which is unsuitable for most soft soldering purposes.
Purkhardhofer (U.S. Pat. No. 2,167,678) teaches an alloy "consisting of the following percentage of metals: bismuth 10%, zinc 42%, tin 22%, lead 23%, cadmium 2.95%, silver 0.05%", His claim is specific in the composition, which consists of several components with a substantial amount of zinc. This is a material absolutely deleterious in normal soldering aims.
Bouton (U.S. Pat. No. 2,303,194). The compositions presented by Bouton do not contain any bismuth and antimony, and cover concentrations of 14-22% cadmium and up to 10% tin. These compositions yield alloys having a melting temperature gap by between 80.degree. to 120.degree. C.
Mulligan (G.B. Pat. No. 185,012) claims readily fusible alloys, having melting points between 155.degree. F. and 320.degree. F. (71.degree.-160.degree. C.). These alloys are useless as soldering alloys, being unsuitable because of a multitude of reasons (brittleness, for example). Furthermore, they are low in lead, but high in bismuth and tin, thus increasing the tin intermetallic compounds formation.
Nomaki (U.S. Pat. No. 4,106,930) is mainly concerned "to provide a process for the direct application of a solder alloy to difficulty solderable materials having an oxide surface at a temperature lower than about 180.degree. C.". His compositions contain a deliberate addition of zinc, which is absolutely deleterious for many purposes.
Rudodobiv I Metallurgie, V. 22, 1967, pp. 46 (Library of Congress TN275A1RT8) teaches how specific soldering alloys containing 15% Sn and up to 10% Cd might replace bearing materials. No teaching can be remarked from this reference regarding other soft soldering compositions.
Goldschmidt (U.K. Pat. No. 186,058) teaches bearing metal alloys, all containing from 15 to 22% antimony and a few other components.
Bouton (U.S. Pat. No. 2,303,193) claims soldering alloys containing 0.5-3% cadmium. In addition, his alloys contain 0.1-1.5% copper, something which generally should be avoided, anyway, because of the deleterious influence upon the flow of the molten solder.
Moranne (U.S. Pat. No. 4,231,794) claims solder alloys containing up to a maximum of 2% tin. These alloys have melting ranges from 248.degree. C. upward, which might be useful for some specific purposes.
The above brief review reveals the high interest which the soft soldering arose among researchers for a long time, looking for specific composition posessing improved properties. It is the object of the present invention to provide such new soft soldering compositions based on Pb-Sn. The reduced tin content is beneficial in lowering the intermetallic layer growth, thus improving reliability, while maintaining essentially the wetting and strength properties. Another benefit of the present invention is to provide cheaper compositions, with their related advantages.